The present invention generally relates to semiconductor devices and more particularly to a semiconductor light-emitting device such as a laser diode.
A DFB laser diode has a diffraction grating formed along an active region thereof and achieves optical amplification or laser oscillation by distributed feedback action from the diffraction grating caused by Bragg diffraction. Such a DFB laser diode has an advantageous feature of stable device operation as compared with a laser diode of Fabri-Perot type, which has a pair of mirrors at axial ends. Thus, DFB laser diodes are used extensively in recent optical telecommunication systems.
In such a DFB laser diode having a diffraction grating, there is caused a refractive index modulation or periodical refractive index change in the diffraction grating, and the optical intensity becomes maximum at the axially central part of the laser diode due to the strong Bragg reflection from the diffraction grating. Because of this, there is a tendency that a non-uniform carrier distribution is induced inside the laser diode when the laser diode is driven to produce large output power. Associated with this non-uniform optical intensity distribution, the DFB laser diode tends to cause higher mode laser oscillation.
Meanwhile, there is a demand of providing a high-reflective film on a rear end of such a DFB laser diode for increasing the optical output power obtainable from the laser diode. However, existence of such a high-reflective film provided on a DFB laser diode deteriorates the stability of the laser oscillation significantly because the high-reflective film causes optical feedback action added to the nominal optical feedback action from the diffraction grating. Further, such a DFB laser diode suffers from the problem of unstable laser operation when a strong reflection beam comes in from an optical fiber. It should be noted that such a strong reflection beam provides additional and unwanted optical feedback action.
In view of these problems associated with a DFB laser diode having a Bragg diffraction grating and associated refractive index modulation, there is proposed a so-called gain-coupled DFB laser diode for realizing a single-wavelength laser diode having improved mode stability. A gain-coupled DFB laser diode is a DFB laser diode having a gain modulation structure, wherein the gain modulation structure forms a gain distribution such that the gain of the laser diode changes periodically in the direction of the optical cavity with a Bragg period.
Various structures are proposed so far for such a gain modulation structure, including a structure in which the thickness of the active layer or an optical guide layer is changed periodically, a structure in which current blocking patterns are formed periodically along an active layer, or a structure in which optical absorption patterns are provided adjacent to an active layer periodically.
Among others, the structure in which the number of quantum wells in an active layer of a multi-quantum structure is changed periodically has an advantageous feature in that the phase of gain coupling and the phase of refractive index coupling coincides with each other and that there occurs no extraneous optical absorption. Thus, the present invention focuses on a gain-coupled DFB laser diode in which the number of the quantum wells in the multiple quantum well structure is periodically changed.
FIGS. 1A and 1B are diagrams showing the construction of a conventional gain-coupled laser diode wherein FIG. 1B shows a part of FIG. 1A in an enlarged scale.
Referring to FIG. 1A, the gain-coupled laser diode is constructed on an n-type InP substrate 11 acting also as a cladding layer and includes an active layer 12 formed of multiple quantum well (MQW) structures A and B on the InP substrate 11 as represented in FIG. 1B. On the active layer 12, there is provided a cladding layer 13 of p-type InP, and a p-type electrode 14 is provided on the cladding layer 13. Further, an n-type electrode 15 is provided on the bottom surface of the substrate 11.
As represented in FIG. 1B in detail, the MQW structure B is formed so as to extend uniformly and continuously in the axial direction of the laser diode, while the MQW structures A are formed intermittently with a period set to be equal to the Bragg period, and as a result, the optical gain is modulated periodically in such an active layer 12 with the Bragg period. Here, it should be noted that the MQW structures A constitute a gain region 12A in which the gain modulation takes place. Further, it should be noted that there is formed a buried region 12B in a gap between a pair of adjacent gain regions 12A, and the gap thus formed is filled with an undoped InP layer having a large bandgap.
It should be noted that such a gain-coupled DFB laser diode 10 shows excellent mode stability over a conventional refractive-index-coupled DFB laser diode because of the gain coupling caused in the periodical gain regions 12A.
In the gain-coupled DFB laser diode 10 of FIGS. 1A and 1B, on the other hand, the buried region 12B of the InP buried layer has a generally smaller refractive index as compared with the gain region 12A, and because of this, there is also caused a refractive index modulation with the period of the diffraction grating, in addition to the gain modulation.
Thus, in the laser diode 10, there is caused not only the desired gain coupling but also a substantial amount of refractive index coupling similar to the case of conventional DFB laser diodes. Thus, when the laser diode is operated to provide large optical output power, there may be caused the problem of mode hopping in which the operational mode of the laser diode jumps to higher modes similarly to the case of conventional DFB laser diodes.
In such a gain-coupled DFB laser diode, therefore, the proportion of the gain-coupling coefficient over the refractive-index-coupling coefficient provides a profound effect on the mode stability and it is necessary to increase the proportion of the gain-coupling coefficient over the refractive-index-coupling coefficient as much as possible. In the case the contribution of the refractive-index-coupling could be minimized, the laser oscillation wavelength coincides the Bragg wavelength, which in turn is determined by the period of the gain regions. Thereby, significant mode stability would be achieved.
In view of such problems of conventional gain-coupled DFB laser diodes, the U.S. Pat. No. 5,452,318 proposes a gain-coupled laser diode 20 having a construction represented in FIG. 2.
Referring to FIG. 2, there is formed an optical confinement layer 22 of n-type GaInAsP on a substrate 21 of n-type InP via an intervening buffer layer 21A of n-type InP, and an active layer 23 is formed on the optical confinement layer 22 such that a gain region 23A of an MQW structure is repeated with a Bragg period in the active layer 23 in the axial direction of the laser diode. In each of the MQW structures constituting the gain region 23A, there is formed a quantum well layer having a bandgap Eg1 as the optical emission layer in which stimulated emission takes place.
In the structure of FIG. 2, there is formed an optical confinement layer 24 of p-type GaInAsP, and a buried layer 25 of p-type GaInAsP having a bandgap Eg3 larger than the bandgap Eg1 of the quantum well layer (Eg1<Eg3) is formed so as to fill the gap between a pair of adjacent gain regions 23A. In such a construction, in which the gain region 23A and also the buried region 23B filled with the buried layer 25 are formed of GaInAsP, it becomes possible to minimize the refractive index coupling.
In the structure of FIG. 2, on the other hand, it should be noted that a p-type GaInAsP layer 25A having a bandgap Eg2 larger than the bandgap Eg3 (Eg3<Eg2) is interposed between the optical confinement layer 22 and the buried region 23B, so as to concentrate the electric current to the gain region 23A and to block the current path of the electric current flowing directly to the optical confinement layer 22 from the buried region 23B.
The laser diode 20 of FIG. 2, while having an improved stability as compared with the laser diode 10 of FIGS. 1A and 1B, has a problem, particularly in the case of reducing the refractive index difference between the buried region 23B and the gain region 23A, in that the bandgap Eg3 of the buried region 23B has to be reduced to a value close to the bandgap Eg1 of the gain region 23A (Eg1≈Eg3). In such a case, however, the laser beam produced by the gain region 23A is absorbed by the buried region 23B and the efficiency of laser oscillation is deteriorated inevitably.
In order to avoid this problem of optical absorption, it is necessary to increase the bandgap Eg3 of the buried region 23B over the bandgap Eg1 of the gain region 23A, while such an increase of the bandgap Eg3 over the bandgap Eg1 causes the problem of increase of the refractive index difference between the gain region 23A and the buried region 23B, and hence the unwanted increase of the refractive-index-coupling coefficient.
Further, the laser diode 20 of FIG. 2, which uses a quaternary material of GaInAsP for the buried layer 25 for achieving lattice matching with respect to the InP substrate 21, suffers from the problem that the composition of the buried layer 25 tends to become non-homogeneous. Thus, there can be a problem of increase of defect density caused by lattice misfit in the region where the composition of the buried layer 25 is deviated from the lattice matching composition. It should be noted that such increase of the defect density can cause the problem such as degradation in the efficiency of optical emission or reduced lifetime of the laser diode.